1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a vertical alignment mode liquid crystal display device with high-speed response.
2. Description of the Prior Art
As well known in the art, a vertical alignment mode liquid crystal display was proposed to improve the viewing angle and response speed properties of a twisted nematic mode liquid crystal display.
Although not shown in the accompanying figures, in this vertical alignment mode liquid crystal display, a liquid crystal layer formed of liquid crystals with negative dielectric anisotropy is sandwiched between upper and lower electrodes, each having a liquid crystal driving electrode. Also, vertical alignment films are disposed on the inner surfaces of the upper and lower substrates, respectively. And polarizers are attached to the outer surfaces of the upper and lower substrates, respectively, in such a manner that their polarizing axes cross each other.
However, in this vertical alignment mode liquid crystal display, the liquid crystals have refractive index anisotropy due to their a rod-like shape of liquid crystal molecules, and for this reason, the screen image of the display vary depending on its viewing angle. For example, before application of an electric field, all the liquid crystals are aligned in a direction perpendicular to the substrates, and thus, on the front of the screen, complete darkness is achieved but on the side of the screen, light is leaked to cause deterioration in image quality.
Thus, in order to compensate for the deterioration of image quality caused by the refractive index anisotropy of the liquid crystals, there was proposed a structure wherein the electric field is distorted to align the liquid crystals in four directions, thereby improving the viewing angle.
For example, U.S. Pat. No. 6,288,762 discloses a structure in which protrusions serving to distort the electric field are formed on substrates. This structure is shown in FIG. 1.
As shown in FIG. 1, a lower substrate 11 and an upper substrate 12 are disposed opposite one another while interposing liquid crystal molecules 13 therebetween. Protrusions 14 are formed on the inner surface of each of the lower substrate 1 and the upper surface 12.
In this structure, an electric field is distorted around the protrusions 14 upon its application such that the liquid crystals 13 are symmetrically aligned. As a result, there are formed a multi-domain of the liquid crystal molecules. This compensates for the deterioration of image quality caused by the refractive index anisotropy of the liquid crystals.
In another attempt to distort the electric field, there was proposed a method in which a liquid crystal driving electrode with slits is used in place of the protrusions. The structure utilizing this liquid crystal driving electrode is shown in FIG. 2, and the principal of operation of this structure equals to that of the protrusions. In FIG. 2, the reference numeral 21 designates a lower substrate, the reference numeral 22 designates an upper substrate, the reference numerals 23 and 24 designate a liquid crystal driving electrode with silts, and the reference numeral 25 designates slits.
However, in the vertical alignment mode liquid crystal display utilizing the protrusion or slit structure, the protrusions or slits serve as a kind of tilting sources in driving the liquid crystals. As shown in FIGS. 3A and 3B, it can be found that, as the interval between the slits 36 is reduced, namely as the number of the slits is increased, the response time of the liquid crystals is shortened. This phenomenon is likewise observed in the structure having the protrusions.
In FIGS. 3A and 3B, the reference numeral 31 designates a lower substrate, the reference numeral 32 designates an upper substrate, the reference numerals 33 and 34 designate a liquid crystal driving electrode, the reference numeral 35 designates liquid crystals, and the reference numeral 36 designates slits.
However, if the tilting sources are increased in order to shorten the response time of the liquid crystals, namely if the number of the protrusions or slits is increased, the increase of disclination lines on a display screen is caused to reduce transmittance. In addition, in this case, deteriorations in properties, such as increases in response time and driving voltage, are caused.
Meanwhile, in order to solve these problems, a jagged liquid crystal driving electrode was proposed by Fujitsu, Co., Japan, in SID 2001 Digest, page 1066. The structure of this proposed electrode is shown in FIGS. 4A and 4B. Referring to FIGS. 4A and 4B, a liquid crystal driving electrode, i.e., a pixel electrode 43, in a lower substrate 41, is formed into a jagged shape, and an upper substrate 42 is provided with protrusions 44. In this structure, although there is an additional titling source between the protrusion and the slit, this tilting source does not serve to make the liquid crystals symmetrically tilted, so that it does not cause the disclination lines.
However, this structure ultimately has the tilting sources consisting of the protrusions and the slits, which are formed on the lower substrate and the upper substrate, respectively. For this reason, if the complete alignment of liquid crystal molecules upon assembling of the substrates is not achieved, it is difficult for the liquid crystals to be stably driven.